Fine tuning Weber IDFs

[Dingleweed]

Thanks modok. That's what I am thinking. Somehow either turn the whole air metering circuit on sooner, or effectively move holes up towards the top of the tube. For me, it's all in the name of having fun with a hobby and just figuring out how stuff works...
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1962 Dune Buggy, 2332cc, W125, 44x37 wedgeports, 48 IDFs, Rancho Pro Street transaxle

[Tbirdusa]

[jg405i]

I would try f-11 / 150 mains and only change the air bleed for the top end mix / high Rpm . just my two cents. I once had an engine like that and size ,But it drove into the night five years ago only never to come home. Drive it by the seat / and lean is fast at the top end just not all day.

[Dingleweed]

Update:

I bought some F15 emulsion tubes a little over a week ago, but haven’t had a combination of no rain and time to work on the project until today.

Here are a couple pics comparing the F2 to the F15 emulsion tubes. F2 is on the left, F15 is on the right.

F2 outer diameter is 7.5 mm

F15 outer diamter is 8.0 mm

According to the spec sheet, the only difference is their outer diameters. They have the same number of holes, sizes, orientation, etc. But, other than the outer diameters, there are some subtle differences. The 8-hole diagonally drilled holes are different between the two. On the F2 ETubes the diagonal hole pattern is all right along the step. On the F15 tubes, the diagonal hole pattern is just above the step and has a variable distance from the step. If you spin it around in your fingers you can see that hole pattern moves up and down with a variable distance between the step and the holes.

First test was to swap only the ETubes and see what the difference is. Comparison is between 135/F2/150 and 135/F15/150. Initially, just driving around normally (light to 1/2 throttle) there is no difference (as would be expected since this driving demands low fuel flow through the main circuit). For a full rpm full-throttle run, the first noticeable difference is that the A/F ratio was just over 1 to 1/2 point leaner. With it being 1+ point leaner in the mid rpms and about 1/2 point leaner in the upper rpms. The second noticeable difference (and this was a slight difference that you really need to pay attention to) was that the air metering circuit came into full operation sooner than before. With the F2, the air metering circuit came into full operation around 5000 rpm. With the F15, the main air metering circuit comes into full operation between 4000 and 4500 rpm.

So, it seems that the effect of a larger outer diameter is to decrease the cross-sectional area the fuel flows through in the main circuit. Without changing the main jet, this has two effects: (1) an increased restriction to fuel flow (the slight leaning out of the entire rpm range), and (2) the volume of fuel flowing speeds up. The speed of the fuel past the lower emulsion tubes is what pulls the fuel out of the inner bore of the ETube exposing the rest of the holes to air on the inside of the tube.

After re-tuning the carbs with a couple of jet changes, I am pretty happy with the improvement. Now, with my current setup with the F15 tubes (145/F15/150) I have an A/F ratio swing of ~1.5 points from 2500 rpm to 4000 rpm (~11.5 to 13 A/F) and fairly flat after that up to 6000 rpm Then from 6000 to 7000 rpm it drops from 13 to 12. Whereas, with the F2 tube setup (135/F2/150) I had an A/F swing of ~3+ points from 2500 rpm to 5000 rpm (~11 to 14+) and fairly flat to 6000 where it dropped from 14+ to ~12.5.

Overall, it was pretty cool to see the slight tuning differences between the F2 and F15 emulsion tubes...
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1962 Dune Buggy, 2332cc, W125, 44x37 wedgeports, 48 IDFs, Rancho Pro Street transaxle

[flatfour]

[modok]

Excellent info.
My tubes look quite different from yours, I think exactly where the 8 main holes are drilled varies even though it's not intended.
So the air jet kicks in sooner, just like you want, nice.

[modok]

New experimental info about changing e-tubes so thought I'd add it here.
Tried some different emulsion tubes in my 40 IDF, "web-ortos" as I call them.

had them running about perfect with 140 main, 215 air f-11(spanish) tubes.
These e-tubes, like many cheapos, had the holes drilled about 1mm above the step

I found a nice set of italian f-11 e-tubes that had the holes drilled perfectly at the step, so threw those in. I found that they ran WAY richer at from transition to about 1/2 throttle! TOO RICH

Fluke? hmmm I thought about dropping float level and smaller main/air combo.....but instead.....

then threw in some cheap f-9 tubes, these are like f-15 but 8.2 diameter instead of 8mm. Also holes drilled sloppy 1mm high. Noticed main system comes in sooner, and they ran generally leaner at all RPMS. Thought about dropping air jet size but didn't....instead

Soldered the four bottom holes of the nice e-tubes to make f-15, and dropped to 135 main. This leaned out the middle a bit, mid-high was perfect now, but still too rich just after transition.

Took the cheap f-9 tubes and turned down to 8mm diameter, so they are NOW f-15 also. With these & 135 main in the mid-high is still perfect, but too lean at transition

???????????
That's a lot to follow, but short story is exactly how the holes are drilled into the emulsion tube at the step DOES matter.........a lot.

Having the 8 holes drilled sloppy 1mm above the step causes it to run leaner at low-mid flows compared to an otherwise identical emulsion tube with the holes drilled right at the step.

I recall in olden times running float level at 11mm or a bit lower, which is great with the OE emulsion tubes. IMO, the need to run 10mm float level that I hear these days makes sense if you have sloppy holes in your tubes!

That said..........it's not all bad. The sloppy f-11 tube, having four holes below the step makes having the 8 too high not as big a problem, perhaps in the end a better design. I consider putting it back as it was to start.........but I doubt I will hmm, lots of possibilities.... what to try next???

[spectre6000]

This is an excellent thread. I'm glad the lazy-thinking "we'll never know" crowd didn't shut it down. I have nothing to contribute, but this is a subject near and dear to my heart, and I hope to see a good bit more exploration on the subject.
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Jason Hopper

-'58 German Market Deluxe Beetle (in progress)
-'84 M1009 CUCV
-'81 K10
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